1. Field of the Invention
This invention relates to a precision cutting insert and more particularly to an insert for ball-type milling cutter intended for duplicating or copying milling machines.
Known duplicating milling machines utilize semispherical milling cutters consisting of high-speed steel or tungsten carbide, constituting a single unit in the case of small diameters or tungsten carbide inserts hard-soldered to a tool holder in the case of diameters in excess of 10 to 12 mm.
Modern duplicating milling techniques, notably in the field of mock-ups, models, prototypes and press tools in the automobile and aircraft industries require semispherical milling cutters of increasing accuracy from the standpoint of the cutting radius. Since in many cases digit-controlled machine tools are used, the milling cutter evolution is programmed on the basis of a precision radius. In digit-controlled machines it is expected that the milling cutter, due to the high degree of precision of its radius, be capable of imparting the desired degree of precision as to the shape of the milled portion.
2. Description of the Prior Art
Known commercial milling cutters, whether of the type comprising a plurality of cutting faces when a high degree of precision is not a primary requirement, or having two cutting faces when a higher degree of precision is required, are rather inaccurate. Their manufacture is mainly dependent upon the skill of the manufacturing personnel and also of the re-sharpening personnel, this precluding any possibility of obtaining a precision of the order of one-hundredth of mm as frequently demanded, and it is extremely difficult or even impossible to sharpen such milling cutters with this degree of precision during their evolution as a consequence of the milling operations accomplished therewith, for a radial-controlled sharpening operation is based completely on the operator's skill.
Known apparatus proposed for this delicate operation are generally cumbersome and subject to operative plays, so that they are not capable of warranting with a sufficient reliability precision radii, and therefore the semispherical milling cutters thus obtained have a handicraft-like character subject to the manual skill of the operator.
From the methods now currently used it appears that:
the requirement concerning the perfect concentricity of the cutting edges generating in evolution the spherical surface (definition of the sphere) is rather problematical;
the strict observance of the radius value baffles any systematical procedure.